1. Field of the Invention
The present invention relates to an apparatus for controlling the rotating speed of an optical recording medium, and particularly relates to a rotation speed controller of the recording medium having a track with wobbling.
2. Description of the Related Art
There are optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) as information recording media for optically recording and reproducing information. Research and development are in progress for increasing recording density of such recording discs while developing recording and/or reproducing apparatus for use with the optical discs.
FIG. 1 is a perspective view showing one example of the optical disc. A groove track 12 for recording information data and a land track 13 for guiding a light beam such as a laser beam as recording light or reproducing light to the groove track 12 are formed on an optical disc 10. In the following description, for brevity of the explanation, the groove track 12 recording data thereto is simply called the track 12.
Both side walls of the track 12 are formed in a zigzag shape. More particularly, wobbling 15 is performed in the track 12 at a frequency corresponding to a rotation speed of the disc. The wobbling of the track 12 is formed in advance before the shipment of the optical disc.
When recording information data are recorded to the optical disc 10, a wobble signal is extracted from a reading signal (e.g., a radial push-pull signal when a push-pull method is used) outputted from an optical pickup of the recording/reproducing apparatus. The rotation speed of the optical disc 10 is controlled by comparing a wobbling frequency with a predetermined wobble reference frequency.
Signal-to-noise ratio (S/N) of the wobble signal can be improved by increasing amplitude of the wobbling 15 formed in the track 12. However, it is not preferable to increase the wobbling amplitude since reduction of a track pitch is necessary so as to increase the recording density of the optical disc. Therefore, disc capacity is increased by narrowing the track pitch while reducing the wobbling amplitude to a detectable extent. In this instance, it is necessary to use a band pass filter (BPF) having a narrow frequency band so as to remove noise included in the reading signal (e.g., radial push-pull signal) when the wobble signal is extracted.
When the rotation speed of the optical disc 10 is deviated from a prescribed rotation speed, the frequency of the wobble signal lies outside the frequency band of the BPF so that no wobble signal can be extracted. Therefore, for example, a method for controlling the rotation speed by using FG pulses corresponding to a spindle rotation speed of a motor for rotating the optical disc 10 was conventionally used together.
However, there is a problem that such a method is not suitable for the high density disc in view of accuracy. In particular, with respect to a zone constant linear velocity (ZCLV) disc having a recording area divided into plural zones, prescribed rotation speed may be different for every zone and rotation speed must be constant within the zone. For the ZCLV disc, therefore, rotating control with high accuracy for every zone is required. Specifically, if a reading position is slightly deviated in a radial direction, the rotation speed is different so that the frequency of the wobble signal is out of the frequency band of the BPF. Accordingly, wobble signal can be hardly extracted through the narrow frequency-band BPF. It is necessary, thus, to control the rotation speed with high speed and high accuracy even when the reading position is changed slightly in the radial direction.